Sunday, November 30, 2008

The Biggest Problem: We Weren’t There

Of course, there was no one around to record the series of events that created the solar system. But there are a few fragments that survive from those early moments, like the years-old crumbs from behind the sofa, that give us clues to how the planets took shape around the youthful sun. The most important clues to the origin of the solar system are to be found not in the sun and planets, but in those untouched smaller fragments: the asteroids, meteoroids, and some of the planetary moons (including our own). These objects make up the incidental matter, the debris of the solar system, if you will.This solar system debris, though rocky, is not mute. As the English Romantic poet William Wordsworth (1770–1850) wrote of hearing “sermons in stones,” so modern astronomers have extracted eloquent wisdom of a different kind from meteorites as well as “moon rocks.”Atoms are made up of three basic particles, protons and neutrons in the nucleus, orbited by electrons. Most elements exist in different atomic forms, which, while identical in their chemical properties, differ in the number of neutral particles (neutrons) in the nucleus. Deuterium and Tritium are famous radioactive isotopes of the more familiar hydrogen.For a single element, these atoms are called isotopes. Through a natural process called radioactive decay, a specific isotope of one atom is converted into another isotope at a constant and known rate, often over many millions of years, depending on the element and isotope involved.Using a device called a mass spectrometer, scientists can identify the “daughter” atoms formed from the “parent” atoms in a sample, such as a meteorite. If the decay rate of the elements in the sample is known, then the ratio of daughter atoms to parent atoms (called isotopic ratios), as revealed by the instrument, betrays the age of the sample. The results of this dating process have been remarkably consistent. Most meteors and moon rocks (which are the only “bits” of the solar system other than the earth we have been able to study exhaustively ) are from between 4.4 and 4.6 billion years old, which has led scientists to conclude with a high degree of confidence that the age of the solar system is about 4.6 billion years.